[luoguP3606] [USACO17JAN]Building a Tall Barn建谷仓（贪心 + 线段树）

Posted 2020-09-17 蒟蒻zht的博客

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把线段都读进来然后排序，先按右端点为第一关键字从小到大排序，后按左端点为第二关键字从小到大排序。

注意不能先按左端点后按右端点排序，否则会出现大包小的情况，如下：

——————

　　———

　　  —

然后直接线段树搞就行，先求区间最小值，如果大于零就更新统一减1。

 

——代码

 1 #include <cstdio>
 2 #include <iostream>
 3 #include <algorithm>
 4 #define root 1, 1, n
 5 #define ls now << 1, l, mid
 6 #define rs now << 1 | 1, mid + 1, r
 7 
 8 const int INF = ~(1 << 31), MAXN = 100001;
 9 int n, m, ans;
10 int minn[MAXN << 2], add[MAXN << 2];
11 
12 struct node
13 {
14     int a, b;
15 }p[MAXN];
16 
17 inline void swap(int &x, int &y)
18 {
19     x ^= y ^= x ^= y;
20 }
21 
22 inline void push_up(int now)
23 {
24     minn[now] = std::min(minn[now << 1], minn[now << 1 | 1]);
25 }
26 
27 inline void push_down(int now, int len)
28 {
29     if(!add[now]) return;
30     add[now << 1] += add[now];
31     add[now << 1 | 1] += add[now];
32     minn[now << 1] += add[now];
33     minn[now << 1 | 1] += add[now];
34     add[now] = 0;
35 }
36 
37 inline void build(int now, int l, int r)
38 {
39     if(l == r)
40     {
41         scanf("%d", &minn[now]);
42         return;
43     }
44     int mid = (l + r) >> 1;
45     build(ls);
46     build(rs);
47     push_up(now);
48 }
49 
50 inline bool cmp(node x, node y)
51 {
52     return x.b == y.b ? x.a < y.a : x.b < y.b;
53 }
54 
55 inline int query(int x, int y, int now, int l, int r)
56 {
57     if(x <= l && r <= y) return minn[now];
58     if(l > y || r < x) return INF;
59     push_down(now, r - l + 1);
60     int mid = (l + r) >> 1;
61     return std::min(query(x, y, ls), query(x, y, rs));
62 }
63 
64 inline void update(int x, int y, int now, int l, int r)
65 {
66     if(x <= l && r <= y)
67     {
68         minn[now]--;
69         add[now]--;
70         return;
71     }
72     if(l > y || r < x) return;
73     push_down(now, r - l + 1);
74     int mid = (l + r) >> 1;
75     update(x, y, ls);
76     update(x, y, rs);
77     push_up(now);
78 }
79 
80 int main()
81 {
82     int i, j;
83     scanf("%d %d", &n, &m);
84     build(root);
85     for(i = 1; i <= m; i++)
86     {
87         scanf("%d %d", &p[i].a, &p[i].b);
88         if(p[i].a > p[i].b) swap(p[i].a, p[i].b);
89     }
90     std::sort(p + 1, p + m + 1, cmp);
91     for(i = 1; i <= m; i++)
92         if(query(p[i].a, p[i].b, root) > 0)
93             ans++, update(p[i].a, p[i].b, root);
94     printf("%d", ans);
95     return 0;
96 }

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